(a) Field of the Invention
This invention relates to a novel and improved preparation process of .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter abbreviated as ".alpha.-APM") or a hydrohalide thereof.
(b) Description of the Related Art
Regarding the preparation of .alpha.-APM which is a substance useful as a sweetening agent and is recently in a growing demand for its strong sweet taste and sweetness characteristics for good quality, a number of processes have already been disclosed centering on chemical preparation processes. Among these, processes making use of an N-protected-L-aspartic anhydride as a carboxylic acid-activated derivative of L-aspartic acid are dominant. In particular, it is considered to be most advantageous industrially to prepare .alpha.-APM by using N-formyl-L-aspartic anhydride which can be obtained easily in a single step by reacting L-aspartic acid with formic acid and acetic anhydride, because the starting materials can be prepared easily and economically and the preparation process is relatively simple.
The above preparation processes of .alpha.-APM, which employs N-formyl-L-aspartic anhydride, features the use of L-phenylalanine methyl ester as the other starting material as disclosed typically in Japanese Patent Laid-Open No. 1350/1971. After formation of N-formyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester as an intermediate, the formyl group as a protecting group is removed to provide .alpha.-APM. A variety of techniques has been proposed centering on the condensation reaction between N-formyl-L-aspartic anhydride and L-phenylalanine methyl ester, the inhibition of isomers, and the removal of the formyl group.
The processes making use of L-phenylalanine methyl ester as one of starting materials however involve cumbersome procedures in esterifying L-phenylalanine into L-phenylalanine methyl ester so as to carry out the condensation reaction with N-formyl-L-aspartic anhydride. Besides, it has also come to the surface as a result of the present inventors' finding that L-phenylalanine methyl ester is, in a free form, prone to undergo self-condensation and cyclization into 2,5-dibenzyl-3,6-dioxopiperazine. This tendency causes various industrial troubles such as yield reduction and quality deterioration of .alpha.-APM.
As a process for the preparation of .alpha.-APM which makes use of N-formyl-L-aspartic anhydride, it has hence been desired to develop a technique not using L-phenylalanine methyl ester as the other starting material.
As preparation processes not relying upon L-phenylalanine methyl ester, it has been disclosed that .alpha.-APM is prepared by subjecting N-formyl-L-aspartic anhydride to direct condensation with L-phenylalanine in acetic acid to form N-formyl-.alpha.-L-aspartyl-L-phenylalanine, removing the formyl group to obtain .alpha.-aspartyl-L-phenylalanine, and then esterifying the .alpha.-aspartyl-L-phenylalanine in the presence of hydrogen chloride in methanol (Japanese Patent Publication No. 26133/1980); and as an improvement to the esterification, .alpha.-L-aspartyl-L-phenylalanine is brought into contact with a medium composed of hydrogen chloride, methanol and water to esterify same, followed by crystallization of the resulting .alpha.-APM as its hydrochloride in a solid form (Japanese Patent Publication No. 50200/1985).
The former process is however accompanied by a drawback that the esterification reaction does not have high selectivity with respect to the two carboxylic acid groups, an esterification reaction of the .beta.-carboxylic acid group and/or another esterification reaction of both .alpha.- and .beta.-carboxylic acid groups take place to substantial extents in addition to the intended esterification of the .alpha.-carboxylic acid group and the selectivity to .alpha.-APM is hence reduced. On the other hand, the latter process has made it possible to improve the selectivity to .alpha.-APM by conducting the esterification reaction in an aqueous solution of hydrochloric acid so as to have the resultant .alpha.-APM to crystallized out as its hydrochloride outside the system. However, the isolation yield of .alpha.-APM is still as low as 50-60% (based on .alpha.-L-aspartyl-L-phenylalanine). The latter process is therefore not considered to be satisfactory fully in yield.